The present invention relates to an image forming apparatus, such as a printer, a facsimile machine or a copy machine, which is capable of forming a toner image on a recording medium, such as a paper sheet, by the electrophotography technique. More particularly, the invention relates to a fixing device for the image forming apparatus.
Generally, the image forming apparatus for forming a toner image on a recording medium by the electrophotography technique includes a photosensitive member to be driven to rotate, an exposure mechanism for forming an electrostatic latent image on the surface of the photosensitive member, a developing mechanism for developing the latent image into a toner image, a transfer mechanism for transferring the toner image onto a recording medium, and a fixing device for thermally fixing the toner image on the recording medium while allowing the recording medium having the toner image transferred by the transfer mechanism to pass therethrough.
FIG. 27 shows a fixing device of the belt fixing type (JP-A-8-334997).
In the fixing device, a fixing belt 1 extends around a fixing roller 2 driven to rotate and a heating roller 3. An pressure roller 4 is pressed against the fixing roller 2 with the fixing belt 1 being interposed therebetween. A recording medium S on which a toner image T is formed is moved to pass through a pressure contact portion (fixing nip) N therebetween in the direction of an arrow in the figure, whereby the toner image T is fused and permanently affixed onto the recording medium S.
In the fixing device thus constructed, if a peripheral speed difference is present between two rotary members, a toner image on the recording medium passing through the press contact portion between the rotary members is blurred to disturb the image. For this reason, usually, such a method that the rotary members are both driven to rotate is not employed, and one of the rotary members is driven to rotate, while the other rotary member is rotated as a follower. That is, the fixing roller 2 is driven to rotate, whereas the fixing belt 1 follows the fixing roller 2 and the pressure roller 4 follows the fixing belt 1. In case of another type of a fixing device in which a fixing belt is not used, one of a fixing roller and a pressure roller which cooperatively form a nip is driven to rotate, whereas the other of the fixing roller and the pressure roller follows the one.
The heating roller 3 includes guide rings 3a as restricting portions which come in contact with the side ends 1b of the fixing belt 1 to restrict lateral offset of the fixing belt 1.
To prevent such a phenomenon that toner is transferred from the recording medium onto the surface of the fixing belt 1 (called offset phenomenon), the fixing device includes an oil application roller 5 for applying release oil, such as silicone oil, as release agent onto the surface of the fixing belt 1.
A length (as viewed in the belt width) of the oil application roller 5 as a oil application mechanism is longer than the width of the fixing belt 1, so that the oil is applied to the belt 1 over its entire width.
The oil application roller 5 is pressed against the fixing belt 1 at a position other than positions where the fixing belt 1 is wound on the rollers 2 and 3. Therefore, it also functions as a tension applying mechanism for applying a tension to the fixing belt.
In the fixing device, an oil application width by the oil application roller 5 of the release agent application mechanism is longer than the width of the fixing belt 1. Therefore, the oil that has been once applied to the top surface of the fixing belt 1 is easy to flow to the back surface 1a of the fixing belt 1.
Generally, the oil application roller 5 includes a shaft and an oil holding layer, made of, for example, felt, provided around the shaft. The oil application roller 5 is pressed against a rotary member (here, fixing belt or endless belt 1), so that oil is squeezed out of the oil holding layer and it is applied to the rotary member. Because of the structure, a pressing force to the rotary member by the oil application roller 5 at the ends of the roller is larger than that at the central portion. Therefore, the end portions of the roller are more greatly compressed than the central portion thereof, and hence the oil application amount at the end portions of the roller is larger than that at the central portion thereof.
The fixing device of the image forming apparatus is designed such that the width (length in the axial direction) of the rotary member is longer than the width (maximum passing width) of a recording medium of which the passing width is the largest of those media that may be supplied for the image formation, although such a design is a nature choice when considering its function.
For this reason, both ends of the rotary member (including the fixing belt 1), which are not in contact with the recording medium having the maximum passing width, are coated with a relatively large amount of oil, which, however, is not absorbed by or transferred to the recording medium. As a result, a relatively large amount of oil is accumulated there. Where the amount of the accumulated oil is excessive, it creeps to the central portion of the rotary member. Because of the creeping oil, a slip will occur between the driving rotary member and the follower rotary member or between the rotary member and the recording medium. This possibly results in that the toner image on the recording medium is blurred to disturb the image.
In particular, in case where the recording medium is a sheet which does not absorb oil, such as a synthetic resin sheet, the slip is likely to occur.
Further, as described above, a relatively large amount of oil applied to both ends of the surface of the fixing belt 1 is easy to flow to the back side 1a of the fixing belt 1.
The oil that has flowed to the back side 1a of the fixing belt 1 gradually moves to between the fixing belt 1 and the fixing roller 2 as its drive roller. When the amount of the oil moving thereto exceeds a predetermined level, a slip occurs between the fixing roller 2 and the fixing belt 1, and as a result, the fixing operation will be instable.
Accordingly, an object of the present invention is to solve the above problems and to provide a fixing device which eliminates the above-mentioned slip, thereby providing a stable fixing operation.
In the publication, JP-A-8-334997, there is no description about the heating width (heating width as viewed in the width direction of the fixing belt 1) by a heater contained in the heating roller 3.
Oil impregnated in the oil holding layer stays within the layer at normal temperature since its viscosity is high, and never leaks out of the layer. In a fixing operation, the belt is heated, and the viscosity of the oil decreases while it is thermally expanded, so that it leaks out of the layer and flows to the belt.
Accordingly, if in the fixing device of FIG. 27, the heating width by the heater of a heating mechanism is longer than the oil application width by the oil application roller 5 of the oil application mechanism (equal to the width of the fixing belt 1 in FIG. 27), the oil application roller 5 is heated over its entire oil application width (the full width of the fixing belt 1 in FIG. 27), the oil is applied to the fixing belt 1 over its entire width.
If the oil is applied to the fixing belt 1 over the entire oil application width, the oil applied to the end portions of the surface of the fixing belt 1 will flow to the back side 1a of the fixing belt 1.
The oil flowing to the back side 1a of the fixing belt 1 gradually moves to between the fixing belt 1 and the fixing roller 2 as its drive roller. When the amount of the oil exceeds a predetermined level, a slip will occur between the fixing roller 2 and the fixing belt 1 because of presence of the oil. As a result, there is a possibility that a stable fixing operation is not performed.
Accordingly, an object of the present invention is to solve the above problems, and to provide a fixing device which eliminates the above-mentioned slip, thereby providing a stable fixing operation.
In the fixing device of FIG. 27, the oil application roller 5 is pressed against the fixing belt 1 at a position other than positions where the fixing belt 1 is wound on the rollers 2 and 3. Therefore, the oil application roller 5 also functions as a tension applying mechanism for applying a tension to the fixing belt 1. If the length of the oil application roller 5 is shorter than the width of the fixing belt 1, the fixing belt 1 is bent at both ends of the press contact portion 1c by the oil application roller 5 (the bending portions are designated by 1d), as shown in FIGS. 28A and 28B.
Therefore, the following problems arise.
The bending gradually decreases with the movement of the fixing belt 1, and is removed (flattened) when the belt 1 is wound on the roller (in this case, the heating roller 3) located just downstream of the press contact portion 1c. At this time, as shown in FIG. 28C, if the side end 1b of the fixing belt 1 comes in contact with the inner wall 3b of the guide ring 3a, a stress is generated in the bending portions 1d (near the side end 1b) during the flattening of the bending portions 1d. That is, as indicated by an arrow in FIG. 28C, when the belt 1 is wound on the roller 3, the bending portions 1d are turned as indicated by an arrow to be flattened as indicated a phantom line 1dxe2x80x2, and at this time, the side ends 1b of the belt 1 come in contact with the inner walls 3b of the guide rings 3a. However, the bending portions 1d are finally flattened. As a result, a stress is generated in the bending portions 1d (near the side ends 1b).
The stress is large as the bending state of the bending potions 1d at the start of winding the fixing belt 1 around the heating roller 3 is large. With repetition of the stress generation, the vicinal portions of the side ends 1b of the fixing belt 1 are easy to be damaged. This fact was confirmed by us.
Accordingly, an object of the present invention is to solve the above problems and to provide a fixing device which protects the fixing belt from being damaged.
The problem that the vicinal portions of the side ends 1b of the fixing belt 1 is easy to be damaged arises not only when the length of the oil application roller 5 is shorter than the width of the fixing belt 1 but also when a press contact member, such as a cleaning blade or a cleaning pad, which is shorter than the width of the fixing belt 1, is pressed against the fixing belt at a position other than a position where the fixing belt is wound on the roller.
Accordingly, an object of the present invention is to solve the above problem and to provide fixing device which prevent the fixing belt from being damaged even if the press contact member is used.
FIG. 29 is a diagram schematically showing a fixing device disclosed in JP-A-8-334997; FIG. 29A is a front view of the fixing device and FIG. 29B is a plan view showing mainly a fixing nip N.
As shown in FIG. 29A, the rotational center of the heating roller 3 is located downstream with respect to the passing or traveling direction (see an arrow S1) of the recording medium, which passes through the fixing nip N, with respect to a straight line A connecting the rotational centers of the fixing roller 2 and the pressure roller 4, when viewed in the axial direction of the fixing roller 2.
Therefore, a force F2 acting on the fixing roller 2 by a tension of the fixing belt 1 suspended between the fixing roller 2 and a heating roller 3 is directed upstream with respect to the passing direction S1 of the recording medium. Accordingly, the resultant force F3 of the forces F1 and F2 is also directed upstream with respect to the passing direction S1 of the recording medium.
Therefore, as shown in FIG. 29B, an axial line 2a of the fixing roller 2 is deflected in a convex shape toward the upstream side of the passing direction S1, by the force F3.
Accordingly, a transporting force acting on the fixing belt 1 at the fixing nip N is a force F4 which acts, at both sides of the fixing nip N, on the fixing belt 1 so as to cause the fixing belt 1 to move toward the center of the fixing belt 1.
At the fixing nip N, the fixing belt 1 is nipped between the fixing roller 2 and a pressure roller 4. Therefore, a transporting force acting on the fixing belt 1 at the fixing nip N greatly influences the fixing belt 1.
Accordingly, if a force F4 acts, at both sides of the fixing nip N, on the fixing belt 1 so as to cause it to move toward the center of the fixing belt 1 as viewed in its width direction, the fixing belt 1 is likely to be creased through the action of the force F4 at a location downstream of the nip N. When the influence by the crease is still left at the contact portion of the fixing belt 1 with the oil application roller 5, the crease hinders the oil application roller 5 from coming in uniform contact with the fixing belt 1. This causes an irregularity in the oil application on the belt 1.
Thus, the fixing device of JP-A-8-334997 has a disadvantage that an irregularity of the oil application is easy to occur.
A belt fixing device shown in FIG. 30 (JP-A-9-138600) is free from the above-mentioned problem since the oil application mechanism is not included. In the fixing device (FIG. 30), a belt 7 extends around four rollers 6a to 6d. A heater 8 is disposed between a belt portion 7a extending from the roller 6a to the roller 6d, and another belt portion 7b extending from the roller 6d to the roller 6b. The structure of the fixing device is extremely complicated.
Accordingly, an object of the present invention is to solve the above problems, and to provide a fixing device which prevents an irregularity of release agent application with a relatively simple construction.
FIG. 31 shows a fixing device of the belt fixing type disclosed in Japanese patent No. 2813297.
The fixing device includes a circulating, endless fixing belt B suspended between a heating roller R3 containing a main heat source H1 and a backup roller R1, a pressure roller R2 pressed against the backup roller R1 with the endless fixing belt B being interposed therebetween, to form a fixing nip N in connection with the endless fixing belt B, an auxiliary heat source H2 as auxiliary heating mechanism for heating the pressure roller R2, and an oil application roller R4 as an oil application mechanism for applying release oil onto the endless fixing belt B when it is brought into contact with the endless fixing belt B at a position located downstream of the fixing nip N but upstream of the heating roller R3 when viewed in the circulating direction of the endless fixing belt B. A recording medium C having a toner image thereon is moved to pass through the fixing nip N, whereby the toner image is fused and fixed on the recording medium.
Generally, a viscosity of release agent, such as release oil depends largely on temperature. The application amount of release agent on the fixing belt varies with temperature of the release oil application mechanism.
In the fixing device shown in FIG. 31, the oil application roller R4 is located above the auxiliary heat source H2 of the pressing roller R2, but is located downstream (i.e., the right side in FIG. 31) of the fixing nip N in the passing direction (from right to left in FIG. 31). Therefore, radiation and hot air stream from the auxiliary heat source H2 substantially fail to reach the oil application roller R4 since those are interrupted by the backup roller R1 and the endless fixing belt B.
Therefore, the oil application roller R4 is heated mainly by the endless fixing belt B alone. Accordingly, in the initial stage of operation of the fixing device, temperature rise of the oil application roller R4 is slow.
When the recording medium C passes through the fixing nip N, a temperature of the endless fixing belt B at a location where it is in contact with the recording medium C remarkably reduces. Therefore, a great temperature difference is present when viewed in its width direction. The temperature difference reflects on the temperature of the release agent application roller R4. In particular in the early stage of the device operation where the temperature rise of the oil application roller R4 is not satisfactory, the temperature difference is easy to occur in the axial direction o the oil application roller. As a result, the oil application amount becomes different also in the axial direction of the oil application roller R4 or in the width direction of the endless fixing belt B. This results in an irregularity of oil application, and thus an irregularity on the fixed image.
Accordingly, an object of the present invention is to solve the above problems and to provide a fixing device which prevents the irregularity of oil application.
Usually, the fixing device of the other type includes a fixing roller containing a heat source and a pressure roller pressed against the fixing roller. A recording medium is moved to pass through a press contact portion between these rollers, whereby a toner image is fused and fixed on the recording medium.
In the fixing device, one (usually the fixing roller) of the paired rollers is rotatably mounted on the frame, while the shaft of the other roller (usually the pressure roller) is urged to the one roller by an urging mechanism such as a spring, whereby those rollers are pressed on each other. That is, a distance of the shafts of both the rollers is not fixed.
In the case of the fixing device of the type in which the roller is heated, a long time is consumed for the initial heating. There is known a fixing device (belt fixing device) in which the endless belt is heated, whereby the initial heating time is reduced.
FIG. 32 is a diagram showing a model of the belt fixing device disclosed in the publication, JP-A-9-138600.
The fixing device includes a heat-resistant endless belt 6, rollers 7a and 7b for supporting the belt 6 on the inner side thereof, a roller 8 for heating the belt 6, and an pressure roller 9 in contact with the outer peripheral surface of the belt 6. The pressure roller 9 is urged at its shaft 9a toward the roller 7a by a spring 9b, whereby the pressure roller 9 is pressed against the roller 7a with the belt 6 being interposed therebetween. Therefore, a distance between the shafts of both the rollers 8 and 7a is not fixed.
In the fixing device, the pressure roller 9 is driven, by a motor M, to turn in a direction of an arrow xe2x80x9caxe2x80x9d. The belt 6 follows the pressure roller 9, and the rollers 7a, 7b and 8 follow the belt 6. A recording medium having a toner image thereon is moved to pass through a press contact portion (nip) N in the direction of an arrow xe2x80x9cbxe2x80x9d, whereby the toner image is heated and fixed on the recording medium.
FIG. 33 is a diagram showing a fixing device disclosed in JP-A-61-10179.
The belt fixing device includes an endless belt 1 to be heated, an pressure roller 2 pressed against the endless belt 1, a non-rotation mandrel 3 for supporting the inner side of the endless belt 1 at the press contact portion N, and a tube heater 4 for heating the endless belt 1.
The non-rotation mandrel 3 is provided with a cam follower 3a, and a cam C is provided for engagement with the cam follower 3a. A desired pressure is generated at the press contact portion N through the operation of the cam C.
In the fixing device, when the pressure roller 2 is driven to rotate, the endless belt 1 follows the pressure roller 2 to move on and along the non-rotation mandrel 3.
A recording medium having a toner image thereon is moved to pass through the press contact portion N, whereby the toner image is heated and fixed on the recording medium. To prevent such a phenomenon that toner is transferred from the recording medium onto the surface of the endless belt 1 (called offset phenomenon), the fixing device includes an oil application roller 5 for applying release oil, such as silicone oil, as release agent onto the surface of the fixing belt 1.
In the fixing device in which one roller is pressed against the other roller by an urging mechanism, such as a spring, the axis-to-axis distance between the paired rollers is not fixed, and hence axis-to-axis distance varies. Therefore, it is difficult to secure a parallelism deviation between both the shafts of those rollers.
Where the parallelism deviation between both the shafts of those rollers is low, there is created a great deviation between a medium transportation direction FA by the roller A at the press contact portion N and a medium transportation direction FB by the roller B at the press contact portion N. This will crease the recording medium.
Since the axis-to-axis distance varies, when a relatively thick recording medium, for example, passes through the press contact portion between the rollers, the roller B moves so as to separate from the roller A in accordance with a thickness of the recording medium. Accordingly, the pressing force at the press contact portion N and the width W (length in the passing direction of the recording medium) at the press contact portion N do not substantively change regardless of whether a relatively thin recording medium or a relatively thick recording medium passes through the press contact portion.
In contrast, the heat capacity of the relatively thick recording medium is large. To fix the toner image on such a thick recording medium satisfactorily, a great amount of heat is required.
To cope with this, when a relatively thick recording medium passes through the press contact portion, the conventional technique applies a great amount of heat to the recording medium in a manner that a fixing temperature is increased or a fixing speed (transporting speed of the recording medium by both the rollers) is made slow. In this way, a fixing defect of the relatively thick recording medium is prevented.
In other words, fixing conditions (fixing temperature and/or fixing speed) must be changed in accordance with the medium thickness.
In the fixing device shown in FIG. 32, the pressure roller 9 is pressed against the roller 7a by the spring 9b. Therefore, the axis-to-axis distance between the rollers is not fixed, and hence axis-to-axis distance varies. Therefore, it is difficult to secure a parallelism deviation between both the shafts of those rollers.
In the fixing device shown in FIG. 33, the non-rotation mandrel 3 is vertically moved in FIG. 33 through the action of the cam C. Therefore, a distance between the non-rotation mandrel 3 and the pressure roller 2 is not fixed. Accordingly, it is difficult to secure a parallelism deviation between the non-rotation mandrel 3 and the pressure roller 2.
For this reason, as described with reference to FIG. 34, where the parallelism deviation between the roller A (in this case, the roller 7a or the non-rotation mandrel 3) and the roller B (in this case, the roller 9 or the pressure roller 2) is low, there is created a great deviation between a medium transportation direction FA of the belt 6 or 1 (FIGS. 32 and 33) by the roller A at the press contact portion N and that FB by the roller B. This will crease the belt 6 (or 1, the same thing will be applied to the subsequent description) or the belt 6 is easy to be damaged. If the belt 6 is not damaged, a force to move the belt 6 in its width direction (for example, arrow FS direction in FIG. 34) constantly acts on the belt 6. The surface of the belt 6 is easy to deteriorate and its life is reduced.
Further, the axis-to-axis distance varies, and hence fixing conditions (fixing temperature and/or fixing speed) must be changed in accordance with the medium thickness.
Accordingly, an object of the present invention to provide a fixing device which prevents the recording medium from creasing, which does not require changing fixing conditions (fixing temperature and/or fixing speed) in accordance with the medium thickness, and which fixes a good toner image on a relatively thick recording medium.
Another object of the present invention to provide a fixing device which prevents the recording medium from creasing, which elongates the life of the belt, which does not require changing fixing conditions (fixing temperature and/or fixing speed) in accordance with the medium thickness, and which fix a good toner image on a relatively thick recording medium.
A first aspect of the invention is directed to a fixing device including: a first rotary member; a second rotary member contacting said first rotary member and forming a nip in corporation with said first rotary member; and an oil application mechanism, which applies oil to at least one of said first and second rotary members. The first aspect is featured in that a width of oil applied by said oil application mechanism is smaller than a width of said at least one of said first and second rotary members to which said oil is applied.
A second aspect of the invention is directed to a fixing device including: a first rotary member; a second rotary member contacting said first rotary member and forming a nip in corporation with said first rotary member; and an oil application mechanism, which applies oil to at least one of said first and second rotary member; and a heating mechanism, which applies heat to at least one of said first and second rotary members. The second aspect if featured in that a width of heat applied by said heating mechanism is smaller than a width of oil applied by said oil application mechanism.
A third aspect of the invention is directed to a fixing device including: a first rotary member; a second rotary member contacting said first rotary member and forming a nip in corporation with said first rotary member; and a heating mechanism, which applies heat to at least one of said first and second rotary member. The third aspect is featured in that said heating mechanism generates larger heat at a central portion thereof than at lateral end portions thereof.
A fourth aspect of the invention is directed to a fixing device including: a first rotary member; a second rotary member contacting said first rotary member and forming a nip in corporation with said first rotary member; and an oil application mechanism, which applies oil to at least one of said first and second rotary member. The fourth aspect of the invention is featured by an oil absorbing mechanism (170, 170xe2x80x2, 170xe2x80x3), which absorbs oil applied by said oil application mechanism.
A fifth aspect of the invention is directed to a fixing device including: a first rotary member; a second rotary member contacting said first rotary member and forming a nip in corporation with said first rotary member; and an oil application mechanism, which applies oil to at least one of said first and second rotary members. The fifth aspect is featured by a blade, which collects oil applied by said oil application mechanism toward a laterally central portion of said at least one of said first and second rotary members to which said oil is applied.
A sixth aspect of the invention is directed to a fixing device including: a first roller; a second roller; an endless belt suspended between said first and second rollers; and a third roller forming a nip in cooperation with said endless belt and said second roller. The sixth aspect is featured in that a rotational axis of said first roller is located in a downstream side of a traveling direction of a sheet with respect to an imaginary line connecting a rotational axis of said second roller to a rotational axis of said third roller.
A seventh aspect of the invention is directed to a fixing device including: a first roller having restricting portions; a second roller; an endless belt suspended between said first and second rollers; a third roller forming a nip in cooperation with said endless belt and said second roller; and a tension application mechanism, which applies tension to said endless belt. The seventh aspect is featured in that a width of said tension application mechanism is shorter than a width of said endless belt, and said tension application mechanism is located closer to a position at which said endless belt commences separation from said second roller than to a position at which said endless belt commences contact with said first roller.
An eighth aspect of the invention is directed to a fixing device including: a first roller; a second roller; an endless belt suspended between said first and second rollers; a third roller forming a nip in cooperation with said endless belt and said second roller; an oil application mechanism, which applies oil to said endless belt; and a heating mechanism, which applied heat to said third roller. The eighth aspect is featured in that said oil application mechanism is located above said heating mechanism, and said oil application mechanism is located in a downstream side of a traveling direction of a sheet with respect to said nip.
A ninth aspect of the invention is directed to a fixing device including: a first roller; and a second roller contacting said first roller and forming a nip in corporation with said first roller. The ninth aspect is featured in that an axis-to-axis distance between said first and second rollers is fixed.
A tenth aspect of the invention is directed to a fixing device including: a first roller; a second roller; an endless belt suspended between said first and second rollers; and a third roller forming a nip in cooperation with said endless belt and said second roller. The tenth aspect is featured in that an axis-to-axis distance between said second and third rollers is fixed.
Two or more of the features of the first to tenth aspects may be selectively combined together.
The present disclosure relates to the subject matter contained in Japanese patent applications Nos.:
1) Hei. 11-10460 (filed on Jan. 19, 1999);
2) Hei. 11-10462 (filed on Jan. 19, 1999);
3) Hei. 11-45569 (filed on Feb. 23, 1999);
4) Hei. 11-45567 (filed on Feb. 23, 1999);
5) Hei. 11-45566 (filed on Feb. 23, 1999);
6) Hei. 11-45568 (filed on Feb. 23, 1999);
7) Hei. 11-56217 (filed on Mar. 3, 1999);
8) Hei. 11-123081 (filed on Apr. 28, 1999);
9) Hei. 11-137801 (filed on May 18, 1999); and
10) Hei. 11-150028 (filed on May 28, 1999),
all of which are expressly incorporated herein by reference in their entireties.